Let documents {attach} static files. Fixes #1019.

Until now, making static files end up in the same output directory as an article that links to them has been difficult, especially when the article's output path is generated based on metadata. This changeset introduces the `{attach}` link syntax, which works like the `{filename}` syntax, but also overrides the static file's output path with the directory of the linking document.

It also clarifies and expands the documentation on linking to internal content.
On second thought, warning about any overwritten file would create noise every time someone regenerates a site without first cleaning the output directory. I suppose warning about save_as conflicts would require indexing all the static files' save_as paths, and warning about a collision when they are set. I'm not sure it's a common enough case to justify the additional code complexity, though, especially since it will be pretty obvious what happened when the user looks at his newly-written page. In any case, it probably doesn't belong in this patch, since the problem is not a new one.